1. Field of the Invention
The present invention relates to a method and means for graphically recording electrocardiographic data which reduces the amount of redundant data recorded while properly recording atypical events.
2. Description of the Prior Art
The electrical phenomena produced by the physiological functioning of the heart are commonly used for diagnostic purposes. To this end, electrodes are applied at various locations on the body. Leads, connected to the electrodes, are coupled to a galvanometer which detects the difference in electrical potential at the various locations. Typically, the galvanometer drives a pen which displays the potential differences on a moving paper tape in the form of corresponding graphic deflections. The electrocardiogram so produced is visually inspected and analyzed by the cardiologist to obtain the diagnosis. Medically significant features of the electrocardiogram include the duration, magnitude and shape of the voltage deflections. The electrocardiogram is usually retained for reference and comparison purposes. Analysis and storage by a computer is also used.
An electrocardiogram includes a series of voltage deflections for each beat of the heart. A typical or normal heart beat or pulse rate is approximately 70 beats per minute or one beat every 0.8-0.9 seconds (800-900 milliseconds). The cyclical functioning of the heart typically produces a signal complex containing 0.5 seconds of activity which produce potential differences followed by 0.3 seconds of resting in which little or no potential differences are produced.
Since the size, shape and duration of the voltage deflections are extremely important, any workable scheme of electrocardiography must present the electrocardiographic data in a standardized graphic form. This permits comparison of electrocardiograms taken at different times or under different conditions, analysis by different physicians and many other advantages. The format of the paper tape and the recording speed have, therefore, become standardized. The paper tape comprises an elongated chart formed of a plurality of squares. The longer dimension of the chart is used to display time relationships and contains major indicia denoting 0.2 seconds time intervals. The smaller dimension of the grid is used to display voltage magnitudes with major indicia denoting 0.5 millivolt and minor indicia 0.1 millivolt. This chart is run past the galvanometer driven pen at a speed of 25 millimeters per second. To be medically acceptable, any graphic display of electrocardiographic data must correspond to the foregoing format.
There are numerous circumstances in which electrocardiographic data must be recorded over an extended period of time. For example, it may be necessary to compare electrocardiographic data generated by the patient at rest with that generated by the patient under exercise conditions. A standard test of this type including the initial period of normal activity to establish normal electrocardiographic data, the exercise period, and the rest period subsequent to the exercise period, may occupy 10 to 30 minutes. A continuous record of a 10 minute exercise test recorded at the standard speed of 25 mm/sec will produce a paper chart approximately 50 feet long. A 30 minute exercise test would provide an electrocardiogram three times as long.
In intensive cardiac care units, it is necessary to establish even longer term monitoring to include continuous monitoring of all electrocardiographic data for periods of several days.
It will be readily appreciated that the volume of the graphic record produced under these conditions may approach the overwhelming. It becomes impractical for the cardiologist to review this mass of record in detail. As a result, significant, but randomly and/or infrequently occurring unusual phenomena may not be observed. Typical of these phenomena are variations in the rhythms of the heart termed "arrhythmias" which are detected by observing the resulting atypical or aberrant voltage deflections and/or timing in the electrocardiogram.
Heretofore, little opportunity has existed to reduce the size of the recording. While sampling techniques may be employed, there is always the risk that data of medical significance will not be in the sample. Reducing the speed of the chart from the 25 mm/sec standard speed would so change the magnitude and durational relationships of the data with respect to the standardized form as to render such an electrocardiogram useless. Certain features of the electrocardiographic signal might also be lost at reduced recording speed.